This invention relates to the casting of steel strip by a single or a twin roll caster. In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontally positioned casting rolls, which are internally cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a thin cast strip product delivered downwardly from the nip. The term “nip” between the casting rolls is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel, from which it flows through a metal delivery nozzle located above the nip forming a casting pool of molten metal supported on the casting surfaces of the rolls. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting steel strip in a twin roll caster, the casting pool will generally be at a temperature in excess of 1550° C., and usually 1600° C. and greater. It is necessary to achieve very rapid cooling of the molten steel over the casting surfaces of the rolls in order to form solidified shells in the short period of exposure on the casting surfaces to the molten steel casting pool during each revolution of the casting rolls. Moreover, it is important to achieve even solidification so as to avoid distortion of the solidifying shells which come together at the nip to form the steel strip. Distortion of the shells can lead to surface defects known as “crocodile skin surface roughness.” Crocodile skin surface roughness is known to occur with high carbon levels above 0.065%, and even with carbon levels below 0.065% by weight carbon. Crocodile skin roughness, as illustrated in FIG. 1, is known to occur for other reasons. Crocodile skin roughness involves periodic rises and falls in the strip surface of 40 to 80 microns, in periods of 5 to 10 millimeters, measured by profilometer.
We have found that with carbon levels below 0.065% by weight the formation of crocodile skin surface roughness is directly related to the heat flux between the molten metal and the surface of the casting rolls, and that the formation of crocodile skin roughness can be controlled by controlling the heat flux between the molten metal and the surface of the casting rolls. FIG. 2 reports dip tests that illustrates the relationship between the heat flux and the formation of crocodile skin roughness during the formation of the metal shells on the surfaces of the casting rolls in making the thin cast strip. As shown by FIG. 2, we have also found that by controlling the energy exerted by rotating brushes peripherally in contact with the casting surfaces of each casting roll, heat flux between the molten metal and the surface of the casting rolls, and in turn crocodile skin surface roughness on the resulting thin cast strip can be controlled.
This relationship between the heat flux from the molten metal and the surface of the casting rolls and the formation of crocodile skin surface roughness on the thin cast strip has been found to occur whether the casting roll surfaces are smooth or textured. FIG. 3 reports dip tests that illustrate how the heat flux is changed with both smooth and textured casting surfaces on the casting rolls. We have also found that the texture of the casting roll surfaces of the casting rolls change during casting. This change can cause a change in heat flux from the molten metal to the casting roll surfaces and in turn a change in formation of crocodile skin surface roughness on the thin cast strip. We have found a method of directly controlling the formation of crocodile skin surface roughness by controlling the heat flux between the molten metal and the casting roll surfaces, to avoid high fluctuations in the heat flux during the formation of the metal shells during casting and in turn control the forming of crocodile skin surface roughness in the thin cast strip produced.
The energy of the rotating brush against the casting roll may be in turn controlled based on the casting speed by varying the application pressure or the speed of rotation, or both, of an electric, pneumatic or hydraulic motor rotating the brush against the casting surface. The energy of the rotating brush can be measured by measuring the torque of the motor rotating. The heat flux between the molten metal and the casting surfaces of the casting rolls may be initially measured and continually measured, as well as the difference between the real time heat flux and the initial heat flux measured, by measuring the difference in temperature of the cooling water circulated through the casting roll between the inlet and outlet as described in U.S. Pat. Nos. 6,588,493 and 6,755,234. Although this is the best way presently contemplated for measuring the heat flux, the heat flux can be measured by any available method. In any event, by monitoring the heat flux and calculating the difference in heat flux from the initial heat flux measured, the energy exerted by the brush against the casting surface can be automatically controlled by a control system that receives electrical signals from the monitor corresponding to the measured heat flux, and controls the energy exerted by the brush against the casting roll based in the difference in heat flux from the initial heat flux measured.
It was previously proposed to project gas in the casting area adjacent the casting surface to adjust the shape of the crown of the casting rolls. See U.S. Pat. No. 5,787,967. However, it has not been proposed project gas on the casting surfaces of the casting rolls in the vicinity of where brushes remove oxides from the casting surfaces to improve localized heat flux between the molten metal and the casting roll surface in the casting area. The casting area is the area between the casting rolls above the nip where the casting pool is formed. It is the area from the twelve o'clock position on the casting rolls where the seal is formed, typically with gas curtains, as the rotating casting roll surface enter the casting area, and does not include the area adjacent the casting rolls between the discharge of cast strip from the nip and the twelve o'clock position on the casting rolls.
We have delivered gas to the casting surface of the casting rolls to create a gas layer adjacent the casting surface immediately following brushing of oxides from the casting surface. A method of localized control of heat flux in continuous casting of thin cast strip is disclosed that comprises the steps of:                assembling a pair of counter-rotating casting rolls laterally to form a nip between circumferential casting surfaces of the rolls through which metal strip may be cast;        forming a casting pool of molten metal supported on the casting surfaces of the casting rolls above the nip to form a casting area;        assembling a rotating brush peripherally to contact the casting surface of each casting roll in advance of contact of the casting surfaces with the molten metal in the casting pool;        removing oxides from the casting surface of each casting roll by contacting the casting surface of each casting roll with the rotating brush;        delivering gas to the casting surface between the rotating brush and entry to the casting area to form a gas layer on the casting surface of each casting roll where the oxides have been removed; and        counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel toward the nip to produce a cast strip downwardly from the nip.        
The steps of removing oxides from the casting surface of each casting roll and delivering gas on the casting surface of each casting roll may occur simultaneously in the nip between the rotating brush and the casting surface of the casting roll. The gas also, or in the alternative, may be introduced upstream of the rotating brush adjacent the brush. In addition, the step of forming a gas layer may comprise introducing the gas into a housing provided about the rotating brush. Alternatively, the step of forming the gas layer on the casting surface of each casting roll to replace the removed oxides may comprise flooding with a gas the casting surface adjacent rotating brush before entry to the casting area.
The gas may comprise at least one gas selected from the group consisting of nitrogen, argon, hydrogen, carbon monoxide, water vapor, dry air, helium or a mixture of two or more thereof.
The casting surfaces of the casting rolls may be textured with a random distribution of discrete projections. Portions of the projections may or may not extend above the gas layer.
Alternatively, a method of localized control of heat flux in continuous casting of thin cast strip is disclosed comprising the steps of:                assembling a pair of counter-rotating casting rolls laterally to form a nip between circumferential casting surfaces of the rolls through which metal strip may be cast;        forming a casting pool of molten metal supported on the casting surfaces of the casting rolls above the nip to form a casting area;        assembling a rotating brush peripherally to contact the casting surface of each casting roll in advance of the casting area;        removing oxides from the casting surface of each casting roll by contacting the casting surface of each casting roll with the rotating brush;        delivering gas to the casting surface between the rotating brush and entry to the casting area in at least three zones extending along the casting surfaces of the casting rolls to form a gas layer on the casting surface of each casting roll where the oxides have been removed; and        counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel toward the nip to produce a cast strip downwardly from the nip.        
The gas projected in the respective zones may be different in composition, mixture, pressure, or at least two thereof the delivering of the gas on the casting surface. Further, the gas may be projected in at least five zones extending along the casting surfaces of the casting rolls. In any event, the delivering of the gas on the casting surface of each casting roll may be adjacent the nip formed between the rotating cleaning brush and the casting surface of the casting roll. The delivering of the gas to form a gas layer may comprise introducing the gas into a housing provided about the rotating brush. Also, the step of delivering gas to the casting surface of each casting roll to replace the removed oxides may comprise flooding the casting surfaces adjacent the rotating brushes with a gas.
The casting surfaces of the casting rolls are textured with a random distribution of discrete projections. Again, portions of the projections may or may not extend above the gas layer.
The gas nozzles may be capable of delivering in the respective zones different gas compositions, gas mixtures, pressures, or at least two thereof. Again, the gas may comprise at least one gas selected from the group consisting of nitrogen, argon, hydrogen, helium, water vapor, dry air, carbon monoxide, carbon dioxide or a mixture of two or more thereof
Further, an apparatus for continuously casting thin cast strip is disclosed that comprises:                a pair counter-rotating casting rolls having circumferential casting surfaces laterally spaced to form a nip therebetween through which thin cast strip may be discharged downwardly, and capable of supporting a casting pool of molten metal on the circumferential casting surfaces adjacent the nip to form a casting area;        rotating brushes capable of removing oxides from the casting roll surfaces of each casting roll, positioned to remove such oxides from the casting surfaces in an area away from the casting area; and        gas nozzles capable of directing gas on the casting surface of each casting roll between the brush and the casting area to form a gas layer where oxides have been removed from the casting surfaces of the casting rolls.        
The apparatus for continuously casting thin cast strip claimed may have the gas nozzles capable of delivering gas adjacent the rotating brush, and flooding the casting surface of each casting roll adjacent the brush with gas. In addition, a housing may be provided about each rotating brush that also supports at least some of the gas nozzles.
Alternatively, an apparatus for continuously casting thin cast strip is disclosed that comprises:                a pair counter-rotating casting rolls having circumferential casting surfaces laterally spaced to form a nip therebetween through which thin cast strip may be discharged downwardly, and capable of supporting a casting pool on the circumferential casting surfaces adjacent the nip to form a casting area;        rotating brushes capable of removing oxides from the casting roll surfaces of each casting roll, positioned to remove such oxides from the casting surfaces in an area away from the casting area;        a control system capable of measuring and controlling a desired degree of cleaning of the casting surfaces of the casting rolls with a majority of projections on the casting surfaces exposed and provide wetting contact between the casting surface and the molten metal of the casting pool by controlling the energy exerted by the rotating brushes during a casting campaign; and        gas nozzles capable of directing gas on the surface of the casting rolls adjacent the brushes to form a gas layer where oxides have been removed from the casting surfaces of the casting rolls.        
The apparatus for continuously casting thin cast strip may have the gas nozzles capable of directing gas on the surface of the casting rolls to flood the area adjacent the position of the brushes before the casting area.
Alternatively, an apparatus for continuously casting thin cast strip with localized heat flux control is disclosed comprising:                a pair counter-rotating casting rolls having circumferential casting surfaces laterally spaced to form a nip therebetween through which thin cast strip may be discharged downwardly, and capable of supporting a casting pool on the circumferential casting surfaces adjacent the nip to form a casting area;        rotating brushes capable of removing oxides from the casting surfaces of each casting roll positioned in an area away from the casting area; and        gas nozzles capable of delivering gas at the casting surface between the rotating brush and entry to the casting area in at least three zones extending long the casting surface of each casting roll to form a gas layer on the casting surface of each casting roll where the oxides have been removed.        
The gas nozzles may be capable of delivering in the respective zones different gas compositions, gas mixtures, pressures, or at least two thereof. The gas nozzles may be capable of delivering of the gas on the casting surface in at least five zones along the casting surface of the casting roll. Also, the gas nozzles may be capable of delivering of the gas on the casting surface of each casting roll adjacent the nip formed between the rotating cleaning brush and the casting surface of the casting roll. Further, the gas nozzles may be capable flooding the casting surfaces adjacent the rotating brushes with a gas. In addition, a housing may be provided about the rotating brush, and the gas nozzle may capable of delivering the gas to form a gas layer through the housing.
Again, the casting surfaces of the casting rolls may be textured with a random distribution of discrete projections.
The gas may comprise at least one gas selected for the group consisting of: nitrogen, argon, hydrogen, helium, water vapor, dry air, carbon monoxide, carbon dioxide or a mixture of two or more thereof
The apparatus for continuously casting thin cast strip claimed may have in addition a control system that comprises:                hydraulic motors capable of controlling the contact the casting surface of each casting roll in advance of contact of the casting surfaces with the molten metal in the casting area; and        a monitoring device capable of monitoring the torque of the hydraulic motors to control the energy exerted by the rotating brushes against the casting surfaces of the casting rolls using the desired degree of cleaning as a reference to clean the expose a majority of projections of the casting surfaces of the casting rolls and provide wetting contact between the casting surface and the molten metal of the casting area.        
The monitoring device may be capable of monitoring the torque of the hydraulic motors by measuring the pressure differential between inlet and outlet of hydraulic fluid through the hydraulic motors. Alternatively, the monitoring device may be capable of measuring the torque between the hydraulic motor and a chock or a motor mount.